The present invention relates to an abrasive liquid feed apparatus used in a polishing apparatus. More specifically, the present invention relates to an abrasive liquid feed apparatus capable of feeding an abrasive liquid which contains in stable form an additive, such as aqueous hydrogen peroxide. The present invention also relates to a method for feeding an additive to an abrasive liquid feed apparatus, and a polishing apparatus.
With recent rapid progress in technology for fabricating high-integration semiconductor devices, circuit wiring patterns have been becoming increasingly fine and, as a result, spaces between wiring patterns have also been decreasing. As wiring spacing decreases to less than 0.5 microns, the depth of focus in circuit pattern formation in photolithography and the like becomes shallower. Accordingly, surfaces of semiconductor wafers on which circuit pattern images are to be formed by a stepper are required to be polished by a polishing apparatus to an exceptionally high degree of surface flatness. To accomplish such a high degree of surface flatness, it has become common to use a polishing apparatus.
FIG. 4 shows an example of a main part of a polishing unit of the polishing apparatus. The polishing unit comprises a turntable 142 having a polishing cloth (polishing member) 140 adhered to an upper surface thereof, a wafer holder 144 for holding a semiconductor wafer W as an object to be polished, in a manner such that the wafer W is capable of being rotated and pressed against the polishing cloth, and an abrasive liquid feed nozzle 146 for feeding an abrasive liquid Q onto the polishing cloth 140. The wafer holder 144 is connected to a wafer holder shaft 148 and supported in a manner such that it is capable of being vertically moved by an air cylinder (not shown).
The wafer holder 144 has an elastic mat 150 made of, for example, polyurethane, attached on a lower surface thereof. The semiconductor wafer W is adapted to be held by the wafer holder 144, while it makes intimate contact with the elastic mat 150. Further, a cylindrical guide ring 152 is provided at an outer peripheral portion of the wafer holder 144, so as to prevent displacement of the semiconductor wafer W from the lower surface of the wafer holder 144 during a polishing operation. The cylindrical guide ring 152 is fixed to the wafer holder 144. A lower end face of the guide ring 152 projects beyond the lower holding surface of the wafer holder 144. The semiconductor wafer W is held in a recess formed inside the projecting portion of the cylindrical guide ring 152.
With this arrangement, the semiconductor wafer W is held on the elastic mat 150 attached on the lower surface of the wafer holder 144 and, while the wafer holder 144 presses the wafer W against the polishing cloth 140 on the turntable 142, the turntable 142 and the wafer holder 144 are rotated, so as to effect relative movement between the polishing cloth 140 and the semiconductor wafer W, to thereby conduct polishing. During polishing, the abrasive liquid Q is fed from the abrasive liquid feed nozzle 146 onto the polishing cloth 140. As the abrasive liquid, use is made of, for example, a slurry obtained by suspending fine abrasive particles in an alkali solution. Thus, chemical polishing is effected by an alkali solution, and mechanical polishing by abrasive particles.
In order to conduct polishing to a desired extent by using the above-mentioned polishing apparatus, the abrasive liquid is required to be fed to the polishing unit in a stable manner, at a predetermined concentration and at a predetermined feed rate. A system for feeding an abrasive liquid comprises: a crude liquid tank in which a crude liquid in the form of a mixture of powdery silica and, KOH or NH4OH is stored; an adjusting tank for preparing an abrasive liquid having a predetermined concentration by diluting the crude liquid with D.I. (De-ionized) water or a chemical liquid; a supply tank in which the abrasive liquid prepared in the adjusting tank is temporarily stored before the abrasive liquid is fed to the abrasive liquid feed nozzle 146 of the polishing unit; and abrasive liquid feed pipes provided between the tanks and between the supply tank and the abrasive liquid feed nozzle 146 of the polishing unit.
In order to modify a wafer surface to be polished, an additive, such as an oxidant, is added to the abrasive liquid. Illustratively stated, in order to oxidize a metallic film made of copper or tungsten formed on the semiconductor wafer W, an oxidant such as H2O2 (aqueous hydrogen peroxide) is added to the abrasive liquid.
When aqueous hydrogen peroxide is added to the abrasive liquid, the polishing rate of a wafer varies, depending on the concentration of aqueous hydrogen peroxide in the abrasive liquid (when the concentration of aqueous hydrogen peroxide is low, the polishing rate is low). However, the concentration of aqueous hydrogen peroxide in the abrasive liquid inevitably decreases, due to decomposition of aqueous hydrogen peroxide over time. Further, since an amount of an additive to be used is small, feeding of the additive must be highly accurate. Therefore, in order to maintain a polishing rate at a predetermined level, a concentration of an additive, such as aqueous hydrogen peroxide, in the abrasive liquid must be detected during a polishing operation, and maintained accurately at a predetermined level.
In view of these requirements, the present invention has been made. It is an object of the present invention to provide an abrasive liquid feed apparatus, a method for feeding an additive to an abrasive feed apparatus, and a polishing apparatus, in which the concentration of an additive (such as aqueous hydrogen peroxide) in an abrasive liquid can be automatically detected during a polishing operation and maintained accurately at a predetermined level.
The present invention provides an abrasive liquid feed apparatus comprising: a supply tank for storing an abrasive liquid having a predetermined concentration; an additive feed means which feeds an additive to the supply tank; an additive concentration measurement means which measures a concentration of the additive in the abrasive liquid in the supply tank; and a control means which controls the concentration of the additive. When the concentration of the additive measured by the additive concentration measurement means decreases to a first concentration value outside a predetermined range, the control means enables the additive to be fed to the supply tank, to thereby increase the concentration of the additive to a second concentration value within the predetermined range.
The above-mentioned abrasive liquid feed apparatus may further comprise a metering means which measures an amount of the abrasive liquid in the supply tank, and the control means may increase the concentration of the additive from the first concentration value to the second concentration value by calculating an amount of the additive to be fed to the supply tank, based on the concentration of the additive measured by the additive concentration measurement means and the amount of the abrasive liquid measured by the metering means, and enabling the additive in the calculated amount to be fed to the supply tank using the additive feed means.
In the present invention, the additive may be fed to the supply tank in predetermined meter units.
The present invention also provides a method for feeding an additive to an abrasive liquid feed apparatus, which abrasive liquid feed apparatus comprises a supply tank for storing an abrasive liquid having a predetermined concentration, and an additive feed means which feeds an additive to the supply tank. The method comprises the steps of:
measuring a concentration of the additive in the abrasive liquid in the supply tank; and
calculating an amount of the additive to be fed to the supply tank when the concentration of the additive in the abrasive liquid in the supply tank decreases to a first concentration value outside a predetermined range, and feeding the additive in the calculated amount to the supply tank, to thereby increase the concentration of the additive in the abrasive liquid in the supply tank to a second concentration value within the predetermined range.
The above-mentioned method may be carried out such that when the concentration of the additive in the abrasive liquid in the supply tank decreases to the first concentration value, the amount of the additive to be fed to the supply tank is calculated from a residual amount of the abrasive liquid in the supply tank and the concentration of the additive in the abrasive liquid in the supply tank.
The present invention further provides a polishing apparatus comprising a polishing means and the above-mentioned abrasive liquid feed apparatus. The polishing means conducts polishing by holding and pressing an object to be polished against a polishing member, while feeding an abrasive liquid to the polishing member.